1. Field of the Invention
This invention relates to a hot wire air flow meter. More particularly, this invention relates to a hot wire air flow meter suitable for measurement of the flow rate of air taken into an internal combustion engine for a motor vehicle, and also relates to a method of manufacturing such a hot wire air flow meter.
2. Description of the Prior Art
A hot wire air flow meter has a heating coil which is formed as an exothermic resistor that is disposed in an air flow path the flow rate of which is to be measured. In order to eliminate any reduction in the temperature of the heating coil due to the cooling effect of the air flow, a current which flows through the air flow path is increased so as to heat up the coil. The air flow rate is determined from this increase in the current. This type of air flow meter can be constituted without employing any movable parts and, at the same time, it enables direct detection of the mass flow. For this reason, air flow meters of this type are generally adopted to perform air-fuel ratio control in the internal combustion engines of motor vehicles.
The exothermic resistor provided in this type of air flow meter comprises a very thin metal wire, e.g., a platinum wire having a diameter of several tens microns. For instance, an exothermic resistor such as the one disclosed in Japanese Utility Model Laid-Open No. 56-96326/1981 is formed in such a manner that a metal wire provided as an exothermic resistance wire is wound around a core wire, that is, a bobbin made of a ceramic material.
Another type of exothermic resistor is disclosed in an already filed patent application (now U.S. Pat. No. 4,790,182) which is a bobbinless exothermic resistor in the form of a coil formed from a metal wire and overcoated with glass except for opposite end portions which are welded to a support for the exothermic resistor.
In the case of an exothermic resistor formed of a metal wire wound around a core wire or bobbin made of a ceramic material employing one of the above conventional techniques, the quantity of heat heating up the body of the bobbin and the quantity of heat transmitted through the bobbin to the support on which the exothermic resistor rests are not negligible. There is therefore a problem of retardation of the transient response to any fluctuation in the air flow, in particular, resulting in the occurrence of surging when the vehicle is sharply accelerated or decelerated. In addition, it is necessary during the process of manufacturing exothermic resistors to perform a coil winding operation for each exothermic resistor, which makes automatization of the process difficult.
In contrast, the bobbinless type of exothermic resistor has improved response characteristics and can be manufactured with an improved degree of automatization because the coil winding operation can be continuously performed for a plurality of resistors of this type. However, it is difficult to handle the opposite end portions of the wire which are not coated with glass, and there is a problem regarding a reduction in the ease with which the operation of securing the resistor to the support can be performed in the manufacturing process. In addition, the layer of coating glass which acts as a support member for supporting the coiled portion of the wire must have a substantial thickness so as to ensure the specified strength of the final products. In consequence, heat transfer between the wire and the air flow is obstructed due to the glass layer having inferior heat conductivity, thereby causing deterioration in the transient response characteristics.
In the structure of the bobbinless type of exothermic resistor, the inner surface of the cylindrical member which is formed by means of glass coating (in which the wire extends helically) is brought into contact with the outside air. If any dust and/or ionic substances are contained in the air the flow rate of which is to be measured, the dust and/or ionic substances become attached to the inner surface of the cylindrical member, or, in the worst case, the inner space of the cylindrical member becomes filled with accumulated dust. In this worst case, the heat generating from the wire is transmitted through the medium of the dust, thereby impairing the advantage of the bobbinless type. If the inner space of the resistor is filled with attached and accumulated ionic substances, short circuiting takes place between adjacent coiled portions of the wire, and characteristics specific to the exothermic resistor are thereby changed. In a method of manufacturing the conventional bobbinless exothermic resistor, a step of removing a bobbin after a coil has been formed by winding a metal wire around this bobbin is adopted, and chemical etching is utilized as a means for removing the bobbin, thereby necessitating an additional process for performing this etching. This makes the overall manufacturing process more complicated.